Fifty-three or 9% of 594 Taiwan monkeys, Macaca cyclopis, were found infected with a Trypanosome species. The parasites were recovered from monkey blood in diaphasic blood-agar culture medium with a Locke's solution overlay. Trypanosomes were found on only a few thick blood smears and no dividing or multiplying forms were found. Some of the organisms had a free-flagellum (4.7 microns), an undulating membrane and both ends of the body were pointed. The total length averaged 41.7 microns; nucleus was slightly anteriorly located. Blood from culture positive monkey would not infect monkeys or other animals, but cultured parasites readily infected monkeys and one mouse, and one rat treated with cortisone. Triatoma rubrofasciata and Triatoma protracta fed upon culture-positive monkeys did not become infected but one of the former became infected after membrane feeding upon cultured parasites mixed with monkey blood. Studies were also done on the development of Trypanosome conorhini in monkeys and other animals. The parasite was recovered from the hind-and mid-gut of naturally infected Taiwan Triatoma rubrofasciata (109 of 117). Some bugs also had salivary gland infections. Most laboratory animals (rats, mice, Mongolian gerbils, guinea pigs, rabbits) developed infections detected by blood-smear and culture. The parasite was serially passaged through monkeys and bugs monthly for one year but there was no change in the development pattern. Only trypmastigotes were present in blood smears; no dividing forms were found. Triatoma rubrofasciata and Triatoma protracta readily developed gut infections when fed upon monkeys, but only a few Rhodnius prolixus became infected.(ABSTRACT TRUNCATED AT 250 WORDS)